Image intensifier tubes are used in night/low light vision applications to amplify ambient light into a useful image. A typical image intensifier tube is a vacuum device, roughly cylindrical in shape, and generally includes a body, photocathode and faceplate, microchannel plate (MCP), and output optic and phosphor screen. Incoming photons are focused on the glass faceplate by external optics, and strike the photocathode that is bonded to the inside surface of the faceplate. The photocathode converts the photons to electrons, which are accelerated toward the MCP by an electric field. The MCP has many microchannels, each of which functions as an independent electron amplifier, and roughly corresponds to a pixel of a CRT. The amplified electron stream, emanating from the MCP, excites the phosphor screen and a resulting visible image is passed through output optics to any additional external optics. The body holds these components in precise alignment, provides electrical connections, and also forms a vacuum envelope.
In general, fabrication of a microchannel plate starts with a fiber drawing process, as disclosed in U.S. Pat. No. 4,912,314, issued Mar. 27, 1990 to Ronald Sink, which is incorporated herein by reference in its entirety. For convenience, FIGS. 1-4, disclosed in U.S. Pat. No. 4,912,314 are included herein and discussed below.
In FIG. 1, there is shown a starting fiber 10 for the microchannel plate. Fiber 10 includes glass core 12 and glass cladding 14 surrounding the core. Core 12 is made of glass material that is etchable in an appropriate etching solution. Glass cladding 14 is made from glass material which has a softening temperature substantially the same as the glass core. The glass material of cladding 14 is different from that of core 12, however, in that it has a higher lead content, which renders the cladding non-etchable under the same conditions used for etching the core material. Thus, cladding 14 remains after the etching of the glass core. A suitable cladding glass is a lead-type glass, such as Corning Glass 8161.
The optical fibers are formed in the following manner: An etchable glass rod and a cladding tube coaxially surrounding the rod are suspended vertically in a draw machine which incorporates a zone furnace. The temperature of the furnace is elevated to the softening temperature of the glass. The rod and tube fuse together and are drawn into a single fiber 10. Fiber 10 is fed into a traction mechanism in which the speed is adjusted until the desired fiber diameter is achieved. Fiber 10 is then cut into shorter lengths of approximately 18 inches.
Several thousands of the cut lengths of single fiber 10 are then stacked into a mold and heated at a softening temperature of the glass to form hexagonal array 16, as shown in FIG. 2. The cut lengths of fiber 10 together form a hexagonal configuration. The hexagonal configuration provides a better stacking arrangement.
The hexagonal array, which is also known as a multi assembly or a bundle, includes several thousand single fibers 10, each having core 12 and cladding 14. Bundle 16 is suspended vertically in a draw machine and drawn to again decrease the fiber diameter, while still maintaining the hexagonal configuration of the individual fibers. Bundle 16 is then cut into shorter lengths of approximately 6 inches.
Several hundred of the cut bundles 16 are packed into a precision inner diameter bore glass tube 22, as shown in FIG. 3. The glass tube has a high lead content and is made of a glass material similar to glass cladding 14 and is, thus, non-etchable by the etching process used to etch glass core 12. The lead glass tube 22 eventually becomes a solid rim border of the microchannel plate.
In order to protect fibers 10 of each bundle 16, during processing to form the microchannel plate, a plurality of support structures are positioned in glass tube 22 to replace those bundles 16 which form the outer layer of the assembly. The support structures may take the form of hexagonal rods of any material having the necessary strength and the capability to fuse with the glass fibers. Each support structure may be a single optical glass fiber 24 having a hexagonal shape and a cross-sectional area approximately as large as that of one of the bundles 16. The single optical glass fiber, however, has a core and a cladding which are both non-etchable. The optical fibers 24, or support rods 24, are illustrated in FIG. 3, as being disposed at the periphery of assembly 30 and surrounding the plurality of bundles 16. The support rods are also known as filler fibers.
The support rods may be formed from one optical fiber or any number of fibers up to several hundred. The final geometric configuration and outside diameter of one support rod 24 is substantially the same as one bundle 16. The multiple fiber support rods may be formed in a manner similar to that of forming bundle 16.
The assembly formed when all support rods 24 have been placed around the ends of bundles 16 is called a boule, and is generally designated as 30 in FIGS. 3 and 5.
Boule 30 is fused together in a heating process to produce a solid boule of rim glass and fiber optics. The fused boule is then sliced, or diced, into thin cross-sectional plates. The planar end surfaces of the sliced fused boule are ground and polished.
In order to form the microchannels, cores 12 of optical fibers 10 are removed, by etching with dilute hydrochloric acid. After etching the thin plates, the high lead content glass claddings 14 remains to form microchannels 32, as illustrated in FIG. 4. Also, support rods 24 remain solid and provide a good transition from the solid rim of tube 22 to microchannels 32. After the plates are etched to remove the core rods, the channels in the plate are metalized and activated.
The current method of manufacturing an MCP also includes dicing the boule at an angle into thin wafers to produce a bias angle. The wafers are then etched, hydrogen fired to form a conduction layer, and metalized to provide electrical contact. After the boule is sliced into wafers, each wafer is handled individually. A typical size of the wafer is approximately 1 inch diameter.
The microchannels of an MCP each form a generally straight bore extending from input to output surfaces of the MCP. As shown schematically in FIG. 11, MCP 110 includes input surface 111 and output surface 112. The microchannels, designated as 113, are inclined at a bias angle with respect to the opposing input output surfaces. However, each microchannel forms a bore that is substantially centered about a straight axial line extending between the input and output surfaces.
Curved microchannels have been considered as a way of increasing gain of an MCP. Such curved channels have been very tricky and expensive to produce. No known MCP is produced with curved channels, although curved channel electron multipliers have been produced for testing purposes. Two methods are known for making a curved channel MCP. Both methods are described below with respect to FIGS. 6 and 7.
The first method for making a curved channel MCP is shown in FIG. 6. As shown, MCP 63 is heated and placed between two horizontally sliding plates, top plate 61 and bottom plate 62. Each plate is notched to receive approximately one-half of the height of MCP 63. The top and bottom plates are brought together to completely nestle the MCP. Next, the top plate is slid horizontally with respect to the lower plate. This causes shearing of one end surface of the MCP with respect to the other end surface of the MCP, thereby providing curves to the microchannels. This method requires exceptional temperature control, very accurate movement of the shearing plates, and probably does not produce adequate uniformity for an imaging application.
The second method of making a curved MCP is shown in FIG. 7. As shown, MCP 73 is sandwiched between two heated plates 71 and 72. The two closed plates are spun in a counter-clockwise direction (for example). The spinning of the plates produces a centripetal force which pushes the center of the MCP outward. With the exterior surfaces of the MCP fixed by the notches in plates 71 and 72, it is believed that the result is curved channels in the MCP. Like the first method, this method requires accurate temperature control. This method also substitutes the difficulty of high-speed rotary motion for the problem of high accuracy linear motion. It will be understood, however, that the goal of each of these methods is higher gain, and not reduced light transmission.